The present invention relates to a sterilizing unit for a pourable food product packaging machine.
Machines for packaging pourable food products, such as fruit juice, wine, tomato sauce, pasteurized or preserved (UHT) milk, etc., are known in which the packages are formed from a continuous tube of packaging material defined by a longitudinally sealed strip.
The packaging material has a multilayer structure comprising a layer of paper material coated on both sides with layers of heat-seal material, e.g. polyethylene. For aseptic packaging of preserved products such as UHT milk, the packaging material comprises a layer of barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn coated with another layer of heat-seal plastic material defining the inner face of the package contacting the food product.
Aseptic packages are produced by unwinding the strip of packaging material in steps off a reel and through a sterilizing unit, where it is sterilized, for example, by immersion in a chamber of liquid sterilizing agent such as a concentrated solution of hydrogen peroxide and water.
The strip is then fed into an aseptic chamber where the sterilizing agent is evaporated by heating; and the strip is then folded into a cylinder and sealed longitudinally to form, in known manner, a continuous vertical longitudinally sealed tube. That is, the tube of packaging material forms an extension of the aseptic chamber, and is filled continuously with the pourable food product and fed to a forming and (transverse) sealing unit for forming the individual packages, and which grips the tube between pairs of jaws for transversely sealing the tube into pillow packs.
The pillow packs are separated by cutting the sealing portion in between the packs, and are then transferred to a final folding station where they are folded mechanically into the final shape.
More specifically, the sterilizing unit referred to above comprises a chamber containing the sterilizing agent through which the strip is fed continuously. The sterilizing chamber conveniently comprises two parallel vertical branches connected to each other at the bottom to define a U-shaped path, the length of which depends on the traveling speed of the strip to allow enough time to process the packaging material. To effectively process the packaging material in a relatively short space of time, e.g. about seven seconds, and so reduce the size of the sterilizing chamber, the sterilizing agent must be maintained at a high temperature, e.g. of about 70xc2x0 C. In known sterilizing units, this is normally achieved by forming the walls of the sterilizing chamber with a first gap, which is filled, in use, with water recirculated through a thermostatically controlled electric heater.
Whereas the packaging material is completely impermeable to the sterilizing agent on the polyethylene-coated faces of the strip, the layer of paper material is exposed along the edges of the strip and fairly absorbent; and what is known in the trade as xe2x80x9cedge wickingxe2x80x9d (edge absorption) is maintained within acceptable limits providing the strip is kept a limited length of time in the sterilizing chamber, as is the case when the machine is operating normally.
Should the machine, however, be arrested for any reason, the sterilizing chamber must be emptied immediately. Otherwise, the sterilizing agent penetrates the edges of the paper layer and, if the edge portion is penetrated to a width of a few millimeters, inevitably impairs subsequent longitudinal sealing of the strip to form the tube of packaging material as described above.
Following stoppage, and particularly when restarting the machine after a short stoppage, edge wicking tends to occur anyway on known machines, despite the sterilizing chamber being emptied.
In-depth research into the phenomenon has revealed several causes:
the porosity of the paper material: for production cost reasons, this can only be reduced to a certain extent;
hydrostatic pressure : this is also difficult to reduce, in that, being dependent on the necessary processing time, the height of the U-shaped sterilizing chamber can only be reduced by altering the structure of the sterilizing unit, which obviously involves complications in terms of the system as a whole; and
the temperature of the sterilizing chamber during stoppage, and of the sterilizing agent when fed back into the chamber. In particular, edge wicking has been found to be seriously affected by any difference, even of only a few degrees, between the temperature inside the chamber during stoppage and the temperature of the sterilizing agent fed back into the chamber. In known machines, such a difference in temperature is caused by the temperature of the chamber tending to rise once the chamber is emptied during stoppage, on account of the inevitable delay in the thermostat responding to the reduction in thermal absorption when the chamber is emptied. As a result, the temperature inside the chamber typically increases to at least 80xc2x0 C., so that the sterilizing agent remaining on the walls of the chamber and in the paper material tends to evaporate, thus producing saturated vapour inside the chamber so that the pores of the paper material contain a saturated air/vapour mixture.
When the liquid sterilizing agent is fed back in at a lower temperature (due to dissipation, the temperature of the liquid fed back into the chamber is at best a few degrees lower than the liquid inside the chamber when the machine was arrested), the temperature of the strip and therefore of the air-vapour mixture inside the pores is reduced: the effect of this reduction is practically negligible as regards the air, which undergoes a reduction in volume of only a few percent, but is considerably more serious as regards the vapour, which condenses to assume, in the liquid state, a much smaller volume. This drastic reduction in volume produces a strong xe2x80x9csuctionxe2x80x9d effect, which draws the sterilizing agent into the pores of the paper material, and which is the main cause of the edge wicking phenomenon.
By way of a solution to the problem, which is less serious during prolonged stoppages, due to evaporation of the residual liquid and a reduction in relative humidity inside the chamber, sterilizing units have been devised in which, before being fed into the sterilizing chamber, the sterilizing agent is heated by circulating it inside a second gap located outside the walls of the sterilizing chamber and enabling heat exchange with the water inside the first gap.
Use is also made of an auxiliary heat exchanger using cold water from the mains as coolant to cool the water controlling the temperature of the sterilizing chamber before starting the machine, i.e. before feeding the sterilizing agent into the chamber. As the water, however, can only be cooled after preheating the sterilizing agent, which is preheated using the same water as heating fluid, restarting the machine takes a fairly long time, typically as long as 7-8 minutes, which in some cases is even longer than the actual downtime of the machine.
In view of the extremely high output rate of packaging machines, such holdups result in considerable cost in terms of lost production.
It is an object of the present invention to provide a unit for sterilizing strip packaging material for a pourable food product packaging machine, designed to eliminate the aforementioned drawbacks typically associated with known machines.
According to the present invention, there is provided a sterilizing unit as claimed in claim 1.